Methods and materials for transferring heat at the interface between a heat-dissipating component, which typically includes various electronic components in semi-conductor devices, to an external heat dissipator or heat sink are well-known in the art. In this regard, such electronic components generate sufficient heat to adversely affect their operation unless adequate heat dissipation is provided.
According to contemporary methodology, the typical solution to such heat dissipation problems is to provide an external heat dissipator or heat sink coupled to the electronic device. Such heat sink ideally provides a heat-conductive pathway from the heat dissipating component to structures such as fins or other protuberances having sufficient surface area to dissipate the heat into the surrounding air. To facilitate such heat dissipation, a fan is frequently utilized to provide adequate air circulation over the fins or protuberances.
However, essential to any effective system for removing heat from an electronic component to a heat sink requires efficient and uniform heat transfer at the interface between the component and the heat sink. Among the more efficient means by which heat is transferred across the interface between the component and the heat sink has been the use of heat conductive pads. Such heat conductive pads are typically pre-formed to have a shape or footprint compatible with a particular electronic component and/or heat sink, such that a given pad may be easily applied thereto prior to coupling the heat sink to the electronic component.
Exemplary of such contemporary pad-type thermal interfaces are THERMSTRATE and ISOSTRATE (both registered trademarks of Power Devices, Inc. of Laguna Hills, Calif.). The THERMSTRATE interface comprises thermally conductive, die-cut pads which are placed intermediate the electronic component and the heat sink so as to enhance heat conduction there between. The THERMSTRATE heat pads comprise a durable-type 1100 or 1145 aluminum alloy substrate having a thickness of approximately 0.002 inch (although other aluminum and/or copper foil thickness may be utilized) that is coated on both sides thereof with a proprietary thermal compound, the latter comprising a paraffin base containing additives which enhance thermal conductivity, as well as control its responsiveness to heat and pressure. Such compound advantageously undergoes a selective phase change insofar as the compound is dry at room temperature, yet liquefies just below the operating temperature of the great majority of electronic components, which is typically around 50xc2x0 C. or higher, so as to assure desired heat conduction. When the electronic component is no longer in use (i.e., is no longer dissipating heat), such thermally conductive compound resolidifies once the same cools to room temperature.
The ISOSTRATE thermal interface is likewise a die-cut mounting pad and utilizes a heat conducting polyamide substrate, namely, KAPTON (a registered trademark of DuPont) type MT. The ISOSTRATE thermal interface likewise is a proprietary paraffin-based thermal compound utilizing additives to enhance thermal conductivity and to control its response to heat and pressure.
The process for forming thermal interfaces according to contemporary methodology is described in more detail in U.S. Pat. No. 4,299,715, issued on Nov. 10, 1981 to Whitfield et al. and entitled a METHODS AND MATERIALS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE; U.S. Pat. No. 4,466,483, issued on Aug. 21, 1984 to Whitfield et al. and entitled METHODS AND MEANS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE; and U.S. Pat. No. 4,473,113, issued on Sep. 25, 1984 to Whitfield et al., and entitled METHODS AND MATERIALS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE, the contents of all three of which are expressly incorporated herein by reference.
As is well-known in the art, by providing a thermally conductive compound that is formulated to have selective phase change properties (i.e., having a melting point such that the compound is solid at room temperature, but liquefies at or below the operating temperature of the electronic component to which it is coupled) advantageously enables the compound to be easily used and handled when applied to the interface between the component and a given heat sink. On the other hand, by assuming a liquid state when exposed to the operating temperature of the electronic component, such thermally conductive composition advantageously is then able to fill the voids created by air gaps at the interface between the electronic component and the heat sink. Once filled, such gaps no longer impose an impediment to efficient heat transfer. As those skilled in the art will appreciate, heat flow across the interface improves substantially with better mechanical contact between the electronic component and the heat sink.
Despite their general effectiveness at transferring heat, however, many thermally conductive compounds currently in use have the drawback of migrating, or shifting away from the interface surfaces upon which they are intended to be applied. In this regard, phase change thermal interface materials tend to be very sensitive materials that can be easily ablated as such materials are handled in manufacturing and shipping processes. Specifically, due to the generally wax-like nature of such thermal interface materials, such materials are inherently susceptible to deformation and mis-shaping even when subjected to minimal handling. As a consequence, such thermal compounds, once ablated or mis-shapen, become substantially compromised as to its ability to transfer heat across an interface. Specifically, such deformation can cause air gaps or voids to form at the thermal interface, which, as a heat conductive medium, is inefficient.
As such, as opposed to being deployed at the time of manufacturer, as would be optimal to minimize expense and expedite manufacturing, such materials must be applied at a later time, typically on-site by the end user. Such processes are well-known in the art to not only be labor intensive, but also messy and difficult to handle. The latter factor is exceptionally problematic insofar as the same often results in an excessive loss of product, particularly with respect to thermal grease and other prior art compositions.
Accordingly, there is a need in the art for a thermally conductive compound for transferring heat from a heat-dissipating component to a heat sink that is more rugged and durable than prior art composition. There is additionally a need in the art for a thermally conductive interface compound that can be readily deployed in remote manufacturing processes and thereafter withstand the abuse and mishandling typically encountered during shipment. There is further a need for such a compound that is effective in filling the voids between and transferring heat away from a given heat-dissipating component to a heat sink and is likewise easy to handle and apply, and preferably formulated to assume a selective phase change whereby the compound is in a solid state at room temperature, but liquefies when subjected to higher temperatures just below the temperatures at which electronic devices typically operate. There is still further a need in the art for a thermally conductive interface compound that is of simple formulation, easy to produce, may be designed to have desirable viscous and sag resistant properties, and does not require special handling.
The present invention specifically addresses and alleviates the aforementioned deficiencies in the art. In this regard, the present invention is directed to a thermally conductive compound for facilitating the transfer of heat from a heat dissipating component to a heat sink that, in addition to effectively conducting heat, is substantially more durable and rugged than prior art compositions. In the preferred embodiment, the composition comprises a base of paraffin or, optionally, a blend of paraffin and petrolatum having quantities of graphite particles suspended therewith. The blend of paraffin and petrolatum is preferably formed such that the ratio of paraffin to petrolatum by percent weight ranges between 1.0:0 to 3.0. The graphite particles are preferably present in amounts between 10% by 40% by weight, with 35% being most preferred. The particles are further preferably formed to have generally spherical shapes having a diameter equal to or less than 6 microns. Additional components may be added to the thermal composition of the present invention, for example, a synthetic polymer to impart greater durability. Although not preferred, there may further be provided a thinning agent, such as a polyalphaolefin. Other surfactant materials are also contemplated. In all embodiments though, the composition is preferably formed to have selective phase change properties whereby the composite exists in a solid phase at normal room temperature, but melts, and therefore assumes a liquid phase, when subjected to an elevated temperature, typically at or above 50xc2x0 C., temperatures which are below the levels heat-dissipating electronic components usually operate.
The present invention further comprises a process for formulating the thermally conductive composition of the present invention comprising the steps of melting the paraffin (or blending the paraffin and petrolatum) to form a first admixture, followed by adding a polymer to increase the durability of the composition. Thereafter, a first portion of the graphite particles, which preferably comprises 60%xc2x110% of the total final weight of graphite, is then added to such admixture followed by adding a thinning agent, such as a polyalphaolefin. Thereafter, the second remaining portion of the graphite, which preferably comprises 40%xc2x110% of the total graphite weight, is then added, with the resultant admixture then being sufficiently mixed until the entire portion of graphite particles becomes sufficiently dispersed and suspended therewithin. Optionally, although not preferred, a viscosity-enhancing agent, which preferably comprises fumed silica, may then added and thoroughly mixed therein to form the resultant composition. Such resultant composition may then be applied to thermal interfaces as would conventional thermally conductive compounds. In this regard, the composition of the present invention may be directly excoriated upon the interface surface or may be applied to the respective interface surfaces via a conventional coating rods, whether wire wound or roll formed. Alternatively, it is contemplated that the composition may also be melted down and dispensed or sprayed upon the interface surfaces between a heat-dissipating component and a heat sink.
It is therefore an object of the present invention to provide a thermally conductive interface composition that substantially more rugged and durable than prior art compositions, and may be further formulated to have a desired viscosity and sag resistance.
Another object of the present invention is to provide a thermally conductive interface composition that can withstand abuse in shipment and handling that is likewise exceptionally effective in conducting heat from a heat-dissipating component, such as an electronic element, to a heat sink to thus increase the reliability and life of the component.
Another object of the present invention is to provide a thermally conductive interface composition that can be readily deployed in remote manufacturing processes, as oppose to prior art compositions requiring post-shipment, on-site application.
Another object of the present invention is to provide a thermally conductive interface composition that is capable of providing a highly efficient heat transfer medium, which offers economic advantages for options of eliminating costly heat-dispensing mechanism, such as fans, or toward reduction in the size, weight, and cost of heat sinks.
Another object of the present invention is to provide a thermally conductive interface composition that is clean and easy to use, and further, may be used in solid form, or which is in solid form when used, but which will become fluid when performing its function at elevated temperatures.
Another object of the present invention is to provide a thermally conductive interface composition that is of simple formulation and may be readily made from commercially available materials.
A still further object of the present invention is to provide a process for forming the novel thermally conductive interface composition of the present invention, as well as utilizing the same to transfer heat from a heat-dissipating component to a heat sink.